This invention relates to electro-optic imaging and, more specifically, to electro-optic imaging wherein a nematic liquid crystalline composition is imaged by applying an electrical field.
Recently, there has been substantial interest in the discovery of more useful applications for the class of substances known as "liquid crystals". The name liquid crystals has become generic to liquid crystalline materials which exhibit dual physical characteristics, some of which are typically associated with liquids and others which are typically unique to solids. Liquid crystals exhibit rheological characteristics, such as viscosities, which are normally associated with liquids. The optical characteristics of liquid crystals are more similar to those characteristics ordinarily unique to crystalline solids.
In liquids or fluids, the molecules are typically randomly distributed and oriented throughout the mass of the material. Conversely, in crystalline solids the molecules are generally rigidly oriented and arranged in a specific crystalline structure. Liquid crystals resemble solid crystals in that the molecules of the liquid crystalline compositions are regularly oriented in a fashion analogous to, but less extensive than, the molecular orientation and structure in a crystalline solid. Many substances have been found to exhibit liquid crystalline characteristics in a relatively narrow temperature range; below the temperature range the substances typically appear as crystalline solids, and above that temperature range they typically appear as liquids. Liquid crystals are known to appear in three different mesomorphic forms; the smectic, the nematic and cholesteric. In each of these structures, the molecules are typically arranged in a unique orientation.
In the nematic liquid crystalline mesophase structure, the major axes of the molecules lie approximately parallel to each other, but the molecules are typically not specifically organized in any other fashion.
Nematic liquid crystals are known to be responsive to electrical fields, and have been used in various electro-optic cells and imaging systems, for example as disclosed in Williams U.S. Pat. No. 3,322,485; Freund et al., U.S. Pat. No. 3,364,433; Heilmeier et al., U.S. Pat. No. 3,499,112; and Goldmacher et al., U.S. Pat. No. 3,499,702. Most of the known nematic liquid crystalline light valves and display devices make use of the dynamic light scattering characteristics of layers of nematic liquid crystalline materials which have electrical fields placed across the thickness of the layer.
In the cholesteric structure, the molecules are believed to be arranged in definite layers as in the smectic structure; however, within a given layer, the molecules are believed to be arranged with their major axes approximately parallel in a fashion resembling the structure of nematic liquid crystals. Because the major axes of the molecules in the cholesteric structure are believed to be parallel to the planes of the layers, the molecular layers are very thin. The cholesteric structure derives its name from the fact that materials exhibiting the cholesteric liquid crystalline structure typically have molecules which are derivatives of cholesterol or which are shaped very similarly to molecules of cholesterol. Because of the shape of the cholesteric molecule; in the cholesteric structure the direction of the major axes of the molecules in each of the aforementioned thin layers is displaced slightly from the direction of the major molecular axes in the adjacent molecular layers. When compared to a hypothetical straight line axes passing through a cholesteric liquid crystalline substance and perpendicular to the molecular planes within said substance, the angular displacement of the direction of the molecular axes within each adjacent molecular layer traces out a helical path around the hypothetical straight line axes.
Cholesteric liquid crystals are known to be responsive to electrical fields (see Harper, W. J., "Voltage Effects in Cholesteric Liquid Crystals," in Molecular Crystals, Vol. 1, 1966, pages 325-332). The effects of an electrical field upon a sample of a liquid crystalline substance have typically been observed in a cell comprising a film of liquid crystals sandwiched between transparent electrodes, as disclosed, for example in U.S. Pat. No. 3,804,618 and French Pat. No. 1,484,584. U.S. Pat. No. 3,652,148 to Wysocki et al., discloses the application of an electrical field to transform a cholesteric liquid to a nematic liquid crystalline structure.
Haas et al., U.S. Pat. 3,687,515 discloses an electro-optic system wherein a layer of spontaneously homeotropic textured optically uniaxial nematic liquid crystalline composition with the optic axis normal to the plane of the layer was rendered optically biaxial by the application of an electrical field perpendicular to the uniaxial optic axis. When the field is removed, the composition naturally relaxes back into its optically uniaxial, homeotropic texture. Copending application U.S. Ser. No. 349,497 filed Apr. 9, 1973 discloses the use of electrical fields to drive an optically uniaxial nematic between two orientations approximately 90.degree. apart. Each applied electrical field is applied normal to the optic axis and, hence, the molecular axes of the uniaxial nematic.
Further, the use of aligning agents, including silanes are known in the art for aligning liquid crystalline materials, including the alignment of nematics in either the nematic homeotropic or nematic homogeneous texture. See, for example, F. J. Kahn, "Orientation of Liquid Crystals by Surface Coupling agents," Appl. Phys. Lett., Vol. 22, No. 8, Apr., 15 1973.
The application of electrical fields to nematics has heretofore been limited to applying the electrical field in a direction which is normal to the net dipole moment of the molecules so that the molecules will be re-oriented by the applied field. That is, a nematic having negative dielectric anisotropy has a net dipole moment substantially perpendicular to the molecular axis is substantially normal to the net dipole moment. The net dipole moment aligns with the direction of the applied electrical field and hence the molecular axes of the negative dielectric anisotropy nematics becomes substantially aligned with the applied electrical field. Conversely, positive dielectric anisotropy nematics have net dipole moments substantially parallel to the molecular axes of the nematic. Therefore, positive dielectric anisotropy nematics align in an electrical field with both the molecular axes and the net dipole moment substantially parallel to the direction of the applied electrical field.